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    <title>Document</title>
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    <script>
        // 同步回调函数
        const arr = [1,3,5]
        arr.forEach(item =>{
            console.log(item)
        })//1   3  5
        // console.log([...arr]);//[1,3,5]
        console.log('forEach___back')


        //异步回调函数
        setTimeout(() =>{
            console.log('timeout callback')

        },100)
        console.log('setTimeout()之后')//先输出24行   在输出21行
    </script>
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